Direct Shear Testing

Department of Construction Engineering Advanced Geotechnical Laboratory Chaoyang University of Technology -- Direct Shear Testing -- DIRECT SHEAR TESTING Prepared by Dr. Roy E. Olson on Fall 1989 Modified by Jiunnren Lai on Spring 2004 ______ INTRODUCTION The direct shear device is used to determine failure envelopes for soils. The device is not suitable for determination of stress-strain properties of soils. Various aspects of the device and resulting data have been discussed previously and will not be repeated. Most of this chapter will be written with the assumption that the tests are of the fully drained type. A later section will cover undrained testing. STANDARD SPECIFICATIONS Direct shear testing is covered in ASTM standard D-3080, "Standard Method for Direct Shear Test on Soils under Consolidated Drained Conditions". GENERAL CONFIGURATION OF THE DIRECT SHEAR DEVICE A cross section through a typical direct shear device is shown in Fig. 1. The soil sample has a porous stone at the top and bottom to allow free drainage (impervious plates could be used for "undrained" tests). Above the upper stone is a metal loading cap which is, in turn, subjected to the normal force. The sample and rings are mounted in a tank which is typically filled with water, but will be empty if tests are being performed on "unsoaked" soil. Several device configurations are in use but, typically, the shearing force is applied to the outside of the tank, which is on rollers (Fig. 1). The tank transmits the load to the lower porous stone and hence to the lower part of the sample. Load is then transferred through the shearing surface to the top part of the system and hence to a load measuring device. Most laboratory direct shear tests are performed on samples with widths of the order of two to three inches. However, samples up to a meter in width have been used for soils containing gravel-sized particles (Nonveiller, 1954; Schultze, 1957; Bishop, __). OVERALL REVIEW OF TESTING PROCEDURE Introduction The overall testing procedure will be reviewed first, without covering details, and then each aspect of the test will be considered separately. It is assumed that the test is to be fully drained and the sample is undisturbed and cohesive, and is in a sampling tube. Minor modifications cover other cases. 1 Department of Construction Engineering Advanced Geotechnical Laboratory Chaoyang University of Technology -- Direct Shear Testing -- Fig. 1 Apparatus Schematic Drawing of Direct Shear 2 Department of Construction Engineering Advanced Geotechnical Laboratory Chaoyang University of Technology -- Direct Shear Testing -- Trimming the Sample The soil sample is extruded from the sampling tube. The extruded sample must typically be trimmed to fit into the shear box. The soil cannot conveniently be trimmed directly into most direct shear devices because the shear box is typically too large and heavy to be handled conveniently. Instead, a special trimming ring is used. The trimming ring has a height that is standard for that laboratory. If a thinner sample is desired, then after the soil has been trimmed into the ring and one face has been trimmed, a spacer plate is used on the surface just trimmed, to push the soil up into the ring an appropriate distance, and then the other face is trimmed (Fig. 2). Fig. 2 Trimming a Specimen to a Height Less Than That of the Trimming Ring The trimmings can be used to obtain an initial water content but they tend to dry out so fast that such water contents usually turn out to be significantly too low. It is better to weigh the soil in the trimming ring, subtract out the known weight of the ring, and then dry the sample after the test, being sure not to lose any of the sample. Apparatus Assembly The shear box is then assembled with the top and the bottom halves of the box screwed (or otherwise rigidly attached) together. The inside of the shear box is typically lightly greased to minimize side friction, just as for consolidation tests. The lower porous stone is placed in the shear box. Sometimes spacer disks are placed below this stone to adjust the elevation of its top to accommodate soil samples of different thicknesses. The trimming ring is then carefully aligned with the top of the shear box. Sometimes the trimming ring and top shear box have been machined so the ring fits into the shallow slot in the top of the shear box, to provide proper alignment. The sample is then slowly extruded into the shear box by pressing on its top surface, typically using the top porous stone or a suitable disk. The upper porous stone and loading cap are placed in the shear box, and the system to apply the normal-toad is brought into place and a small normal load (seating load) is applied. Consolidation Stage A dial indicator, or other suitable device for measuring the change in thickness of the sample, is quickly mounted and a zero reading taken. A consolidation pressure is then added to the top of the sample using the load-application system of the apparatus (typically a lever arm or a pneumatic system). The consolidation stage proceeds as for a standard incremental one- dimensional consolidation test. Loads are typically applied with a load increment ratio of one, to minimize problems with soil extrusion. Readings of settlement or expansion are taken as a function of time to allow appropriate calculation of consolidation coefficients and to ensure that the sample has come to equilibrium prior to the start of shear. 3 Department of Construction Engineering Advanced Geotechnical Laboratory Chaoyang University of Technology -- Direct Shear Testing -- Preparation for the Shearing Stage During the consolidation stage, the upper and lower halves of the shear box have been tightly screwed together (Fig. 3) to prevent the soil from extruding out from between the boxes. Typically, only two locking screws are used. Prior to shearing the sample, the upper half of the box is typically raised to provide a small separation between the boxes and ensure that the shearing and normal stresses are actually transmitted through the soil rather than from box to box. The boxes are usually separated before the final shearing stage by removing the locking screws, and then using screws that are threaded through the top box but not the bottom box, to lift the upper box (Fig. 3). lifting screws stainless steel loading cap loading arm two at 180 degree locking screws two at 180 degree top porous stone soil sample upper shear box bottom porous stone lower shear box existing aluminum bottom disk Fig. 3 Assembly Drawing of Direct Shear Box For normally consolidated samples, of the order of 5-8% of the applied load has been transferred into side shear in the, upper half of the box so lifting the box momentarily reverses the side shear and causes a small amount of sample disturbance, thus causing a small amount of additional time-dependent consolidation. Prior to starting the shearing stage, the screws used to lift the top box must be withdrawn so the full applied stress plus the weight of the upper half of the shear box, acts on the soil in the potential failure zone, and another stage of a small amount of consolidation begins. If the top half of the shear box is heavy, and not counterbalanced, and the sample is soft, a significant amount of additional consolidation may result. To minimize the time during this stage of the test, the top half of the box is usually raised and then it is released at once; and then readings of sample thickness are continued until the sample has come back to equilibrium. Shearing Stage The shearing stage is usually performed at a constant rate of deformation. Methods of selecting the deformation rate will be discussed subsequently. A rate is selected and the shearing stage begin. Readings are taken of horizontal displacement, vertical movement of the top cap, and shearing force, as a function of time. Stress conditions in the sample become increasingly uncertain as deformation continues so the test is usually stopped at a horizontal deflection of about 0.25 inch even if the shearing stress has not reached a peak value. 4 Department of Construction Engineering Advanced Geotechnical Laboratory Chaoyang University of Technology -- Direct Shear Testing -- Dismantling Stage When the test is over, the shearing stress is reduced to zero. Equipment for measuring deformations is removed. The normal load is then reduced to zero as quickly as possible and the apparatus dismantled. The soil sample starts to rebound as soon as the normal load begins to decrease so the dismantling stage must be quite rapid if there is any desire to measure the water content at the failure stage. Once the apparatus has been dismantled, the two halves of the shear box are separated. Often, a water content sample is taken from the shear zone, and then the water content sample and the rest of the specimen are dried to obtain a final dry weight (and thus an initial water content). Data Reduction The data are reduced by calculating the normal and shearing stresses, plotting a curve of shearing force vs. horizontal movement, and perhaps plotting change in sample thickness vs horizontal movement. The failure condition is plotted in a Coulomb diagram. There are numerous options in the above procedure, e.g., it may be desired to determine the residual strength as well as, or in place of, the peak strength. These options will be discussed in later sections. SAMPLE SHAPE AND SIZE Direct shear samples are shaped like flat disks or flat rectangular prisms.

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